Class 12 Chapter 2 Chromium Properties Reactions oxidation states of Chromium
Introduction
Chromium (Cr), atomic number 24, belongs to the first row of the d‑block elements. Known for its brilliant colors and multiple oxidation states, chromium exhibits rich chemistry central to both industrial and biochemical processes.
Occurrence & Extraction
Chromium is primarily found as the ore chromite (FeCr2O4) in South Africa, Kazakhstan, and India. Extraction is carried out by roasting chromite with sodium carbonate followed by reduction with carbon:
4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2 Na2CrO4 + C → Cr2O3 + CO Cr2O3 + 2 Al → 2 Cr + Al2O3
Physical Properties
- Silvery‑steel grey metal, lustrous and hard
- High melting point: 1907 °C; boiling point: 2672 °C
- Density: 7.19 g cm⁻³
- Paramagnetic due to unpaired d‑electrons
Chemical Properties
Chromium exhibits both metallic and non‑metallic behavior:
- Reacts with oxygen: Forms protective Cr2O3 layer (passivation).
- Reacts with acids: Dissolves in HCl/H2SO4 evolving H2; in oxidizing acids forms Cr(VI) species.
- Reacts with alkalis: Forms chromite and chromate in hot and concentrated solutions.
Oxidation States
Chromium exhibits oxidation states from –2 to +6, the most common being +2, +3, and +6:
| Oxidation State | Species | Color |
|---|---|---|
| +2 | Cr2+ (chromous) | Pale blue |
| +3 | Cr3+ (chromic) | Green |
| +6 | CrO42– / Cr2O72– (chromate/dichromate) | Yellow / Orange |
Important Reactions
Chromium(III) Hydroxide Formation
Cr3+ + 3 OH– → Cr(OH)3 (green precipitate)
Chromate–Dichromate Equilibrium
2 CrO42– + 2 H+ ⇌ Cr2O72– + H2O
Redox: Cr(VI) to Cr(III)
3 Cr2O72– + 14 H+ + 6 e– → 6 Cr3+ + 7 H2O
Applications
- Stainless steel production (adds corrosion resistance)
- Electroplating (decorative & protective coatings)
- Leather tanning (Cr(III) salts)
- Pigments (chromate/dichromate compounds)
- Wood preservation (sodium chromate)
Conclusion
Chromium’s versatile oxidation states and robust physical properties make it indispensable in alloys, coatings, and chemical industries. Understanding its reactions and redox chemistry is key for applications in materials science and environmental chemistry.
